Hollow fiber membrane blood oxygenators are the current gold standard for blood oxygenation. These oxygenators typically incorporate one of four blood flow path configurations, as summarized in U.S. Pat. No. 5,462,619: (1) longitudinal (axial) flow through an annular bundle (see U.S. Pat. No. 4,975,247); (2) circumferential flow around an annular bundle (see U.S. Pat. No. 3,794,468); (3) transverse flow across a bundle of substantially rectangular cross-section (see U.S. Pat. No. 5,188,801); and (4) radially outward flow through an annular bundle (see U.S. Pat. No. 3,422,008). The specifications of the foregoing are incorporated herein by reference in their entireties.
Although the membrane blood oxygenators based on the above principles are generally acceptable for cardiopulmonary bypass during open heart surgeries, they have a number of problems when they are used for respiratory support over longer durations (e.g., days to weeks). They have a relatively large blood-contacting surface area, a large prime volume, and a large physical size with very limited long-term biocompatibility and durability. The drawbacks of these oxygenators are associated with inherent blood fluid dynamics within these oxygenators, including non-uniform blood flow through the fiber membranes, the existence of laminar boundary flow zones between the blood cells and fiber membranes, and large physical size.
The non-uniform blood flow across the fiber membranes results in hyper- and hypo-perfusion of blood in the flow path. Hyper-perfusion does not have any additional benefit once blood is oxygen-saturated. In order to assure that all blood cells in the hypo-perfusion region are well oxygenated, longer flow paths are needed, thus resulting in extended blood contact with the fiber membrane surfaces and a large surface area of the fiber membranes. When blood flows through fiber membranes, a relatively thick blood boundary layer is developed. The blood boundary layer that is formed increases the resistance to oxygen diffusion to blood cells that are not directly in contact with the fiber membrane surface. The gas transfer efficiency can be significantly hindered by the existence of the boundary layer. Therefore, gas exchange membrane surface areas of 2 to 4 m2 and a large prime volume are typically required to provide the needed gas exchange. The non-uniform blood flow can potentially induce excessive mechanical shear stresses or stasis in the blood flow path in the oxygenators. These are the major contributing factors to blood activation and thrombosis formation, resulting in limited long-term biocompatibility and durability. In addition, the large physical size also limits the wearability for ambulatory respiratory support.
In recognition of the drawbacks related to the boundary layer zones in the foregoing patents, methods to decrease the boundary layer effect have been proposed by increasing the shear rate and/or turbulence of the blood flow path by introduction of secondary flows. The blood is directed to flow perpendicular or at a substantial angle to the fiber membranes. Examples of this type of design include those described in U.S. Pat. No. 4,639,353 (Takemura) and U.S. Pat. No. 5,263,924 (Mathewson), the specifications of which are incorporated herein by reference in their entireties. Takemura describes the arrangement of bundles of hollow fibers perpendicular to the direction of blood flow via a series of flow guide structures. Mathewson describes the integrated centrifugal pump and membrane oxygenator in which the hollow fibers are displaced circumferentially in a ring around an impeller of the centrifugal pump. The blood is pumped through the hollow fibers for oxygenation. One drawback of Mathewson's design is that there exist potential stagnant flow zones between the annular fiber bundle and the outer housing wall.
To overcome shortcomings of the prior art in terms of non-uniform blood flow path and less biocompatibility, a rotating impeller was introduced to generate uniform blood flow through an annular fiber bundle, as described in U.S. Pat. No. 8,496,874. The use of an integrated rotating impeller to achieve uniform blood flow may be beneficial. However, integration of the pump with a blood oxygenator into a system can introduce difficulties for manufacturing and complex flow paths in the integrated system.
In consideration of the limitations of the foregoing devices, there is a need for a compact, efficient and non-traumatic blood oxygenator with a low prime volume and a small surface area of gas exchange membranes.